EP 1 312 488 A2 discloses a blocking device for a sand dosing device having a dosing piston connected to a lifting magnet. In a closed position of an inlet channel of the dosing device, an annular bulge of a seal that is connected with the dosing piston rests on a projection of a housing wall. For releasing the inlet channel, the bulge is lifted off the projection by means of the dosing piston.
U.S. Pat. No. 2,325,441 A describes a sand spreader with a reservoir for sand, the bottom end of which is provided with an opening that is adapted to be closed by means of a cup-shaped valve cap. To this end, the valve cap is connected with a piston of a lifting magnet.
JP 2000-071979 A discloses a spreader with a reservoir and an inlet channel. In the closed position, the inlet channel is closed by a closing piston that is fastened to a piston of an electrically operable lifting magnet.
CH 423860 A describes another sand spreading device for vehicles in which a spreading channel is adapted to be closed by means of an elastically deformable tube. For the spreading of sand, coils are excited, so that a slider or plunger-type armature, respectively, that is fastened to the tube is pulled outwardly contrary to the effect of pressure springs, so that the channel is released.
U.S. Pat. No. 1,990,253 A and DE 418 221 C disclose sand spreading devices with mechanical swinging devices.
Sand spreading systems are particularly used in the case of rail vehicles with drive. In so doing, the sand available in the sand reservoir is fed, via the sand dosing and blocking device, to a conveyor device that is usually actuated pneumatically, and subsequently the sand is conducted via appropriate lines to the gap between the vehicle wheel and the rail or the ground, respectively, so that a slip and slide protection is formed.
The sand dosing devices, in particular the piston-controlled sand dosing and blocking devices known from prior art feature a number of disadvantages that will be listed in the following.
The installation of the known, in particular piston controlled sand dosing and blocking devices is basically only possible externally of the sand reservoir, which increases the space requirement of the sand dosing device substantially. Due to the installation height required, only a comparatively small filling volume of the sand reservoir can be provided in the case of a limited installation space. This results in shorter or more sand re-filling intervals.
It is a disadvantage that, with the known sand dosing and blocking devices, the amount of sand to be discharged can moreover be dosed with comparatively little accuracy only, since the amount of sand transmitted to the conveying device is strongly influenced by the sand quality, i.e. in particular the size of the grains of sand. It is especially the setting of small amounts of sand that is possible in a limited manner only with the known piston-controlled sand dosing and blocking devices. The smallest dosing piston opening gap and hence also the smallest setting of the amount of sand depends on the grain size of the sand used, so that the largest grain of sand determines the smallest dosing piston opening gap or the smallest amount of sand, respectively.
During operation of the sand spreading system, in the case of pneumatic sand conveyance, a negative pressure is produced by the injector function by the suction effect, said negative pressure having a negative effect in particular in the case of small settings of the amount of sand or a small dosing piston opening gap, respectively, since the sand is compacted in the outlet cross-section and functional disorders, e.g. congestions, may be caused. Even by particular additional measures (e.g. the controlled supply of an air flow into the injector chamber for reducing the negative pressure via a bypass) can this effect be compensated for partially only.
The setting of the amount of sand can only be performed individually for every single sand dosing device, for instance, by a setting screw at the actuator which is positioned externally. This means that every single sand dosing device has to be adjusted to the amount of sand required with a correspondingly high effort of time and personnel.
Furthermore, the sand dosing devices known have no useful possibility of continuously regulating the amount of sand conveyed as a function of the velocity of the vehicle.